Academic journal article The American Biology Teacher

Teaching Basic Lab Skills Using Diverse Microbial Communities in a Biologically Relevant Context

Academic journal article The American Biology Teacher

Teaching Basic Lab Skills Using Diverse Microbial Communities in a Biologically Relevant Context

Article excerpt


Traditionally, introductory laboratory courses that use microbiological techniques have a medical focus and tend to use pure cultures to teach basic lab skills (Leboffe & Pierce, 2006). Consequently, students get comparatively little exposure to the dynamics of microbial communities, especially in natural biological contexts. However, microbial species often live in complex multispecies communities in both medical and environmental contexts. This laboratory exercise provides students the opportunity to learn basic lab skills while working with diverse microbial communities. Specifically, students examine the colonization of leaf material in streams by naturally occurring communities of microorganisms. This colonization plays a significant role in ecosystem dynamics because it facilitates energy flow and nutrient cycling in streams (Graca, 1993).

When leaves first enter streams they undergo leaching that releases dissolved organic and soluble inorganic materials from them (Webster & Benfield, 1986). The leaves are then colonized by multiple species of bacteria and fungi, often forming microbial biofilms (Barlocher & Kendrick, 1975; Suberkropp & Klug, 1976; Graca, 1993). Biofilms, which are common in nature, are microbial communities covered in exopolysaccharide material; they are often visible as slimy films on wet surfaces (e.g., on wet rock surfaces near the stream's edge; Atlas & Bartha, 1998). Biofilms form as a sucession of different organisms colonize the leaf-litter surface. Within hours of falling into the stream, leaf-litter particles are colonized by bacteria, and other groups of microorganisms appear later. Temperature and other environmental factors influence the rate at which the leaf litter is colonized (Atlas & Bartha, 1Uf8).

Microorganisms colonizing leaf litter use enzymes to break down complex molecules in the leaf, such as cellulose, chitin, and lignins (Webster &Benfield, lia6). Bacteria involved in leaf decomposition include the Actinomycetes (e.g., Streptomyccs, Nocardwides, Pseudonocardia, Nocarlia, and Micromonospora) and bacteria from the Cytophaga-Flavobacterium-Bacteroidetes group (Wohl & McArthur, 1998; Lydell et al., 2004). Fungi involved in leaf decomposition include aquatic hyphomycetes such as Clavariopsis aquatica, Tricellula aquatica, Tripospermum camelopardus, and Gyoerffyella rotula (Descals, 2005; Gulis et al., 2005). These microorganisms play a critical role in the degradation of leaf material by both digesting the leaf matter and increasing the palatability of the leaf material to macro-invertebrates (e.g., isopods and amphipods; Webster & Benfield, 1986). In addition, these microorganisms produce biomass that is consumed by other organisms in the community. Thus, microbial colonization facilitates the decomposition of leaf material and provides food for other organisms within the ecosystem.

In the exercise presented here, students measure colonization of leaf material by both bacteria and fungi during the first 3 days of leaf submersion using serial dilution, aseptic technique, bacterial plate spreading, and both fungal and bacterial staining to quantify microbial colonization of leaf material. This exercise can be completed in two lab sessions, is relatively inexpensive, and does not require any prior experience with microbiological techniques. In addition, both qualitative and quantitative approaches can be used to describe the data, which gives instructors the opportunity to modify the exercise according to the statistical expertise of the students.

To date, we have used a somewhat recipe-based approach to run this exercise. Here, we introduce a more inquiry-based approach, in which students are given the opportunity to generate new hypotheses and design experiments after completing the initial experiment (see Discussion). It may also be possible to modify the experiment so that a more inquiry-based approach is used for the entire exercise. …

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